It's a tie between Belmont's Revenge and Harmony of Dissonance for me.
Puwexil put it well regarding Belmont's Revenge.
Normal mode in HoD is a little boring due to it being too easy, but in hard mode you are faced with a better challenge, so I mostly just play hard mode. I love the eerie atmosphere throughout the game, the soudntrack(quality and all, I feel the soundtrack really adds to the experience of HoD), the enemy designs, a lot of the bosses, the subweapon and magic system... It's just a great game to me.